Preservation of homeostasis is a fundamental condition for any organism. The most conservative mechanism for cellular protection is the expression of heat shock proteins (hsp), which are involved in the repair and stabilization of key cellular processes after stress. Although the primary function of hsp is circumscribed to intracellular events, they have been found outside cells, released by an active process. We hypothesize that extracellular hsp are exported to alert the immune system that a localized stress or injury has occurred. Therefore, the immune system is primed to mount a timely response in case the localized insult should propagate. We have coined this systemic mechanism to sense stress the stress observation system (SOS). An important feature of the SOS is that hsp are released associated with extracellular vesicles (ECV) derived from the plasma membrane. These vesicles contain information for targeting specific cell types for the delivery of the stress information. Prior investigations have shown that Hsp70 (Hsp72), the major inducible form of the hsp family, was found embedded in the plasma membrane of cells recovering from a stress. In addition, Hsp70 can be inserted into artificial lipid bilayers, openin ion conductance pathways. Moreover, Hsp70 was released from cells associated with ECV. Hsp70-positive ECV is able to interact with macrophages (M s), inducing a response that primes cells to ameliorate, prevent, or defend the organism from subsequent insults, which is consistent with the role of hsp in stress tolerance. The objective of this application is to elucidate the mechanisms of Hsp70 insertion into the plasma membrane and ECV release and interaction with M s. These investigations will provide novel cellular mechanisms for protein export and activation of immune cells, which are likely to constitute new pillars of knowledge for cellular biology as well as biomedical research. Moreover, our studies may define a new regulatory system that senses the occurrence of stress in the form of vesicles that permit the communication between distant cells. An understanding of this novel communication system may be of help in the diagnosis and treatment of critically ill patients.